The present invention relates to glycol-based antifreeze compositions, particularly to corrosion-inhibiting compositions useful in automotive service.
Current progress in automotive technology is leading to the development of more efficient, smaller, lighter cars. In one aspect, this progress has resulted in the increased use of aluminum in engine cylinder heads and in components of the engine cooling system such as radiator cores and water pump housings and fittings. In another aspect, this progress gas resulted in engines designed to operate at higher temperatures. Overall, these and other factors have recently placed more stringent demands upon the performance of the aqueous glycol-based "antifreeze" solutions employed as engine coolants. Antifreeze formulations must not only prevent the automotive engine cooling system from freezing and protect against overheating but must also provide corrosion protection of cooling system surfaces with which it is in contact. Conventional antifreezes which have been formulated with corrosion inhibitors satisfactory for protection of engines having cast iron, steel, solder, copper and brass component surfaces are generally not suitable in preventing the corrosion of aluminum and its alloys. This is particularly true under increased operating temperatures. In the cooling system, aluminum corrosion products, particularly aluminum oxide, typically circulate to and deposit on internal radiator surfaces interfering with the heat transfer necessary to keep the engine from overheating.
Of course, various corrosion inhibitors, particularly silicate compounds, are known as components of antifreeze formulations which aid in protecting aluminum. It has, however, been found difficult to formulate such aluminum corrosion inhibitors into a produce which satisfies all of the criteria of antifreeze service. For instance, the silicates are often unstable, particularly in combination with inhibitors necessary for protection of other metals. Instability, as evidenced, for example, by precipitate formation or by loss of corrosion inhibiting activity, may be a problem in terms of shelf life of an antifreeze concentrate or in terms of use life in aqueous solutions under service conditions in the cooling system. In service, the inhibitors must remain stable in the presence of contaminants, such as decomposition products of the glycol upon which the antifreeze is based and substances such as carbon dioxide and exhaust gases which leak into the cooling system, and must remain capable of counteracting the corrosive tendencies of these contaminants. Although stable and otherwise compatible inhibitor combinations have been developed, they are, as a rule, either unduly expensive, or objectionable from the standpoint of toxicology and environmental concerns, or unable to provide fully satisfactory multi-metal corrosion protection, or characterized by unacceptable physical properties.
With regard to features of the invention relating to antifreeze compositions containing certain specified corrosion-inhibiting ingredients, particularly phosphate and silicate compounds, it is generally true that each such ingredient is known to the art for antifreeze service. The present invention, however, is directed to a particular combination of specific ingredients in specific proportions in a novel antifreeze composition which satisfies the needs of the progressing automotive technology. The composition has a relatively high measure of stability in service and provides unexpectedly superior protection of both aluminum and other metals under high temperature service conditions. Of particular importance to the invention is a glycol based composition combining both a moderate amount of silicate component with a relatively large amount of phosphate component. Known antifreezes comprising large proportions of phosphate do not have satisfactory performance with respect to multi-metal corrosion protection. It has been suggested (U.S. Pat. No. 4,242,214) that the presence of the phosphate ion in antifreeze composition is a cause of the corrosion of aluminum surfaces. Moreover, known antifreezes comprising both phosphate and silicate components (e.g., those disclosed in U.S. Pat. No. 4,210,549 and U.S. Pat. No. 3,121,692) do not contain the high level of phosphate found to be critical in the composition of the invention for aluminum protection at high temperature.